The principle of magnetic induction technology for wireless data communication is basically that of an inductively coupled transformer. This inductively coupled transformer has a very low coupling factor, i.e., the mutual inductance is very small. The transmitter side of the inductively coupled transformer is explained with reference to FIG. 1.
FIG. 1 is a block diagram of a circuit 100, comprising a driving voltage source 102, and a series connection of a capacitor 104, an inductance 106 and a resistor 108. The antenna is formed by inductance 106, implemented by an external coil that is used for both transmitting and receiving, e.g., on a time-division multiplex basis. An RLC tank is formed by resistor 108, capacitor 104 (both on-chip), and external coil 106. Node 110 provides a signal voltage representative of the signal transmitted via inductance 106. The tank needs to have a center frequency equal to the channel frequency. When circuit 100 is used as a transmitter, the tank needs to efficiently transmit an adjustable level. The most power-efficient way to do this is using a non-linear class-D amplifier. As known, a class-D amplifier is a power amplifier wherein the output stage is operated in an on/off mode. Advantages of a class-D amplifier include small size, low power dissipation and a high power-conversion efficiency. The high efficiency is based on having the output stage never operate a linear region. Instead, the output devices are either on or off. Since the RLC-tank is a band-pass filter, only the fundamental signal of the square wave is transmitted. A typical modulation scheme used in the magnetic link is binary frequency-shift keying (FSK). As known, FSK is a frequency-modulation technique wherein the signal is used to modulate a carrier wave by shifting the frequency between values determined in advance. The link transmits digital data at a bit-rate of 300 kbit/s. The modulation frequency can be, e.g., 10.6 MHz or 13.56 MHz. The modulation index is unity, meaning that there is 300 kHz between the two tones that represent a digital zero and a digital one. During a transmission of a single bit one of two frequencies is transmitted, depending on the bit's value.